CAT5273 Datasheet, PDF (9/14 Pages) ON Semiconductor – Dual 256‐position I2C Compatible Digital Potentiometers (POTs)

CAT5271, CAT5273
I²C BUS PROTOCOL
The following defines the features of the I²C bus protocol:
1. Data transfer may be initiated only when the bus is
not busy.
2. During a data transfer, the data line must remain
stable whenever the clock line is high. Any
changes in the data line while the clock is high
will be interpreted as a START or STOP condition.
The device controlling the transfer is a master, typically a
processor or controller, and the device being controlled is the
slave. The master will always initiate data transfers and
provide the clock for both transmit and receive operations.
Therefore, CAT5271/CAT5273 will be considered a slave
device in all applications.
START Condition
The START condition precedes all commands to the
device, and is defined as a high to low transition of SDA
when SCL is high. The CAT5271/CAT5273 monitors the
SDA and SCL lines and will not respond until this condition
is met.
STOP Condition
A low to high transition of SDA when SCL is high
determines the STOP condition. All operations must end
with a STOP condition.
DEVICE ADDRESSING
The bus Master begins a transmission by sending a
START condition. The Master then sends the address of the
particular slave device it is requesting. The seven most
significant bits of the 8-bit slave address are fixed as
0101111 for the CAT5271. For CAT5273 the first five bits
are fixed as 01011, and the next two bits are
pin-programmable device address bits (AD1 and AD0). The
next bit (R/W) selects between the type of the instruction
Read or Write. If the bit is logic high, then a Read instruction
is performed. If the bit is logic low, then the Write command
is executed.
After the Master sends a START condition and the slave
address byte, the CAT5271/CAT5273 monitors the bus and
responds with an acknowledge (on the SDA line) when its
address matches the transmitted slave address.
Acknowledge
After a successful data transfer, each receiving device is
required to generate an acknowledge. The Acknowledging
device pulls down the SDA line during the ninth clock cycle,
signaling that it received the 8 bits of data.
The CAT5271/CAT5273 responds with an acknowledge
after receiving a START condition and its slave address. If
the device has been selected along with a write operation, it
responds with an acknowledge after receiving each 8-bit
byte.
When the CAT5271/CAT5273 is in a READ mode it
transmits 8 bits of data, releases the SDA line, and monitors
the line for an acknowledge. Once it receives this
acknowledge, the CAT5271/CAT5273 will continue to
transmit data. If no acknowledge is sent by the Master, the
device terminates data transmission and waits for a STOP
condition.
WRITE OPERATION
In the Write mode, the Master device sends the START
condition and the slave address information to the Slave
device. After the Slave generates an acknowledge, the
Master sends the instruction byte. After receiving another
acknowledge from the Slave, the Master device transmits
the data to be written into the wiper register. The CAT5271/
CAT5273 acknowledges once more and the Master
generates the STOP condition.
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